The term "superconductivity" is applied to the phenomenon of immeasurably low electrical resistance exhibited by materials. Until recently superconductivity had been reproducibly demonstrated only at temperatures near absolute zero. As a material capable of exhibiting superconductivity is cooled, a temperature is reached at which resistivity decreases (conductivity increases) markedly as a function of further decrease in temperature. This is referred to as the superconducting onset transition temperature or, in the context of superconductivity investigations, simply as the critical temperature (T.sub.c). T.sub.c provides a conveniently identified and generally accepted reference point for marking the onset of superconductivity and providing temperature rankings of superconductivity in differing materials. The highest temperature at which superconductivity (i.e., zero resistance) can be measured in a material is referred to as T.sub.o.
As herein employed the terms high superconducting onset transition temperature, high critical temperature and high T.sub.c are employed interchangeably to indicate a material having a superconducting onset transition temperature of greater than 80.degree. C.
Agostinelli et al U.S. Ser. No. 359,306, filed May 31, 1989, titled CONDUCTIVE ARTICLES AND PROCESSES FOR THEIR PREPARATION, commonly assigned, now U.S. Pat. No. 4,950,643, (European patent application 0 334 093, published Sept. 27, 1989, corresponding) discloses electrically conductive articles in which a crystalline heavy pnictide mixed alkaline earth copper oxide is formed on a substrate. The articles exhibit high critical temperatures and superconductivity when the substrate presents a perovskite crystal structure as a coating surface (e.g. a strontium titanate coating surface) or a magnesia or alumina coating surface. To avoid reduction of T.sub.c by less compatible substrates Agostinelli et al proposes the use of barrier layers between the crystalline superconductive oxide layer and the substrate. Suggested barriers include magnesia, a group 4 metal oxide or silicide (particularly zirconia), silver, gold or a platinum metal.
What the art has been sought without success are articles in which superconductive copper oxide coatings are present on supports of non-precious metals, particularly period 4 metals.
In Applicants' concurrently filed, commonly assigned patent application, U.S. Ser. No. 475,527, titled ELECTRICALLY CONDUCTIVE ARTICLE (I), an article is disclosed comprised of support, a barrier and an electrically conductive heavy pnictide mixed alkaline earth copper oxide crystalline coating. The support is an austenitic metal alloy of chromium and one or more group VIII period 4 metals, and the barrier is comprised of a high density hafnia layer.